Windows/Linux/Mac OS X: If you have ever donated your computer's idle cycles to a charitable project, Foldit will be right up your alley. After installing the game you are given protein samples to untangle. On the surface it appears as though you are trying to solve a puzzle of tangled cords, but your solutions are much more valuable:

The number of different ways even a small protein can fold is astronomical because there are so many degrees of freedom. Figuring out which of the many, many possible structures is the best one is regarded as one of the hardest problems in biology today and current methods take a lot of money and time, even for computers. Foldit attempts to predict the structure of a protein by taking advantage of humans' puzzle-solving intuitions and having people play competitively to fold the best proteins.

By leveraging a large pool of users just like distributed computing projects such as SETI@Home, the founders of Foldit hope to build an enormous database of proteins that have been untangled and reduced to their simplest forms by sharp eyed gamers across the world. Below is a video of an extremely good player tearing through a protein puzzle:
Improve your problem solving skills and the world at the same time. Foldit is cross platform with downloads available for Windows, Linux, and Mac OS X.